The present invention relates to a novel photocurable organopolysiloxane composition which is curable with a very small dose of light irradiation with its extremely high photosensitivity and capable of giving a photocured product with high moisture-resistance in the adhesive bonding strength to various substrates.
There have been known hitherto several types of organopolysiloxane compositions curable by irradiation with light, in particular, rich in ultraviolet. These prior art photocurable organopolysiloxane compositions are, however, defective in their relatively poor anti-moisture resistance and strength of adhesive bonding after photocuring so that the cured films thereof come apart from the substrate surface on which they have been photocured, especially, when the cured films are exposed to an atmosphere of high humidity and at a relatively high temperature. For example, the adhesive bonding strength of a cured film of a photocurable organopolysiloxane composition obtained by the application on to the surface of a substrate such as a metal or a ceramic material followed by irradiation with light is greatly reduced by heating it in boiling water for about 1 hour or longer. The photocured film with thus reduced bonding strength to the substrate surface is readily and almost completely removed from the substrate surface when an adhesive tape is applied thereto and then peeled off so that the usefulness of it in the practical use is greatly reduced.